The presence of multiple electronically coupled metal ions in the active sites of metalloproteins imparts unique character to the electronic structures of such systems. The research described in this proposal will determine the extent to which these electronic features play a role in their function. More specifically, this work will examine the influence of electron spin polarization on electron transfer reactions, in particular those relevant to the active sites of several iron-oxo and iron-sulfur metalloproteins. This will be achieved through a confluence of synthetic, physical, and computational chemistries performed on systems ranging from simple coordination compounds to structural models of active sites to surface-modified iron-sulfur proteins. The research will be developed in several stages. First, building on previous results, several model systems will be synthesized that allow for the modulation of electron spin in a controlled and systematic fashion. Time-resolved spectroscopic studies, in conjunction with variable-temperature magnetic measurements, will then be employed to quantify both the electronic structures and electron transfer kinetics of these systems. These studies will provide the first unambiguous experimental data detailing the influence electron spin polarization has on electron transfer reactions involving biologically important metal clusters. More complex systems will then be pursued including, but not limited to, (1) iron-sulfur/porphyrin assemblies to study electron transfer kinetics in models of sulfite reductase; (2) Ru-modified iron-sulfur proteins (e.g., HiPIP) to assess the role of thermally accessible excited spin states within the active-site cluster for protein-based electron transfer; and (3) donor/cluster/acceptor assemblies to effect through-cluster electron transfer, examining the possible role of spin-state modulation for the gating of electron migration in metalloprotein active sites. This work will significantly advance our understanding of mechanistic issues related to metalloprotein electron transfer through its examination of an intrinsic but unexplored aspect of the electronic structure of their active sites.